Optical coherence tomography (“OCT”) is a type of optical coherence analysis that is becoming increasingly popular in research and clinical settings. OCT provides high-resolution imaging of sub-surface features of a sample. This is useful for the in vivo analysis of biological tissues, for example.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. A modern optical coherence analysis technique is termed Fourier domain OCT (“FD-OCT,”) of which there are generally two types: Spectral Domain OCT and Swept Source OCT.
In both techniques, optical waves that have been reflected from an object or sample are combined with reference waves to produce OCT interference signals. A computer produces axial scans (A-lines) or combines many A-lines into two-dimensional cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection by reference to the reference waves.
Spectral Domain OCT and Swept Source OCT systems differ in the type of optical sources and detectors that they use. Spectral Domain OCT systems typically resolve the spectral components of an interference signal by spatial separation. Spectral Domain OCT systems utilize a broadband optical source and a spectrally resolving detector system to determine the different spectral components in each axial scan of the sample. As a result, the detector system is typically complex, as it must detect the wavelengths of all optical signals in the spectral scan band simultaneously, and then convert them to a corresponding interference dataset. This affects the speed and performance of Spectral Domain OCT systems. In contrast, Swept Source OCT systems encode spectral components in time, not by spatial separation. Swept Source OCT systems typically utilize a single tunable laser source that is swept in wavelength over a scan range or band. The interference signal is detected by a non-spectrally resolving detector system.
Swept Source OCT systems often utilize a sampling clock, or k-clock, that is used in the sampling (including resampling) of the interference signals. Basically, the k-clock is used to correct for non-linearities in the frequency sweeping of the swept source. Some Swept Source OCT systems use a hardware-based k-clock to directly trigger the Analog-to-Digital (“A/D”) converter of a Data Acquisition (“DAQ”) system that samples the interference signals. Other Swept Source OCT systems sample the k-clock signals in the same manner as the interference signals, creating a k-clock dataset of all sampled k-clock signals and an interference dataset of all sampled interference signals. Then, the k-clock dataset is used to resample the interference dataset in software. This is also known as a software-based k-clock. The resampling provides data that are evenly spaced in the optical frequency domain, or k-space. This provides maximal SNR and axial imaging resolution for subsequent Fourier transform-based signal processing upon the acquired interference signal spectra or interference dataset.
Fourier transform-based signal processing upon the interference signals provides the “A-line” information, or axial scan depth within the sample at each frequency of the reflected light in the resampled interference dataset. The spatial domain signals that include the “A-line” information, in turn, are compiled from many scans to generate a tomographic image or volume data set.
Because of the potentially high processing overhead that resampling of interference datasets and Fourier transform-based signal processing can incur, manufacturers of FD-OCT systems are increasingly turning to special-purpose processing units such as Field-Programmable Gate Arrays (“FPGA”), and General-Purpose Graphical Processing Units (“GPGPU,” or “GPU”). For more information, see “Scalable, High Performance Fourier Domain Optical Coherence Tomography: Why FPGAs and Not GPGPUs,” Jian Li, Marinko V. Sarunic, Lesley Shannon, School of Engineering Science, Simon Fraser University, Burnaby BC, Canada. Proceedings of the 2011 IEEE 19th Annual International Symposium on Field-Programmable Custom Computing Machines, FCCM '11, 2011.